The invention is intended more specifically for a high bit rate wireless communication network using the LMDS (Local Multipoint Distribution Service) system, which is based on a cellular architecture. In this architecture, a sending/receiving station equipped with antennas to be able to communicate with the other stations of the cell, can serve as a node of the cell. In this case, the architecture is called “P-MP” (Point-MultiPoint). Another possible architecture for this system is the “MP—MP” (MultiPoint-MultiPoint) architecture, in which each station can be a relay in a call between two other stations of the wireless network.
The millimetric frequencies (30 to 3000 GHz) or EHF (Extra High Frequencies) are used with a view to increasing the information transfer rates in the wireless networks. At such frequencies, the available bandwidths are wide (greater than 1 GHz) but the attenuation as a function of distance is high.
The coverage rate is therefore limited by the short range of the millimetric frequency transmit stations that make up such a wireless network, and by the need to have an “LOS” (Line Of Sight) between a sending station and a receiving station of the network. Despite the low cost and the performance of the LMDS systems at millimetric frequencies, their coverage limitations mean that they cannot be deployed intensively.
In an MP—MP architecture, or mesh network architecture, in which each station of the network can be a relay station, the obstacles can be circumvented. Thus, the coverage and the capacity of the high bit rate wireless network are improved.
The attenuation as a function of distance limiting the transmission range between two stations of the high bit rate wireless network is offset by a high antenna gain. Increasing the gain of an antenna involves improving its directivity and therefore concentrating its radiation pattern in a precise direction. Consequently, the alignment of the antenna must also be accurate.
Furthermore, changing the configuration of the network must involve a reliable realignment of the antenna system of the stations of the network with a 360° coverage in azimuth for each station.
A solution proposed by the Radiant Networks company is an antenna system made up of four high gain millimetric antennas. The system uses an access technique known as “TDMA/TDD” (Time Division Multiple Access, Time Division Duplex). In this technique, the time is divided into frames of a fixed duration, which are in turn subdivided into “slots”. The slots are used individually for sending/receiving between two antennas aligned for a call between their respective stations. The antennas are aligned mechanically through the intermediary of a motor. This solution is complex, expensive and bulky. Furthermore, the mechanical alignment is neither reliable nor instantaneous.
Another solution is described in patent application GB2238174A. This document describes a high-frequency antenna made up of a set of dielectric lenses adjacent to one another and arranged to obtain 360° coverage in azimuth. The rear surface of each lens is itself made up of several radiating elements for sending and receiving. These elements are precisely arranged to send or receive beams according to different, evenly spaced angular directions, the periodicity of which is maintained from one lens to the next. The lenses are delimited on each side by a flat surface, the direction of which passes through the central axis of symmetry of the optical system. This antenna system is complicated to implement. In this system, a number of radiating elements are used for the same lens. The result is, necessarily, that certain of these radiating elements are out of focus. The antenna system does not present the same radiation pattern, and in particular the same directivity, in all the directions corresponding to the feeds.